Aggregation of ionic liquids with organic anions, driven by cation–anion interaction strength

•Aggregation and cation–anion interaction force of [C8(MIM)2] with ibuprofenate and ascorbate were evaluated.•CAC value of both ILs was quite low compared to the analogue with bromide.•The IL with ascorbate had a greater cation–anion interaction strength and lower CAC.•The IL with ascorbate presented a better packing of IL monomers at air–water interface.•The lower CAC of IL with ascorbate is due to it can organize itself in a typical ion pair.